Frequency discriminator



Oct. 20, 1942. N. l. KORMAN 2,299,581

FREQUENCY DISCRIMINATOR v Filed Jan. 25, 1941 Z0 40 60 60 Z 0F' Bil/vp07m/Z517 Gttorneg Patented Oct. 20, 1942 FREQUENCY DISCRIIWINATORNathaniel I. Korman, H

addon Heights, N. J.,

assignors to Radio Corporation of America, a

corporation of Delaware Application January 25, 1941, Serial No. 375,901

(Cl. Z50-27) 2 Claims.

'I'his invention relates to frequency discriminators and moreparticularly to a frequency discriminatcr in which the deleteriouseffects of undesired coupling and improper relations of reactance toresistance are minimized.

Frequency modulated currents are not detected in the same manner as anamplitude modulated current. In general, it is necessary to convert thefrequency modulated current into an amplitude modulated current fordetection. Devices capable of such conversion have been called frequencydiscriminators. One type of frequency discriminator is disclosed inUnited States Patent 2,057,640 which issued on October 13, 1936, toFrank Conrad. The Conrad discriminator employs a pair of resonantcircuits upon which the frequency modulated currents are impressed. Oneof the resonant circuits is made to have maximum response at a frequencyabove the midfrequency of the applied currents, while the other resonantcircuit is `made to have maximum response to currents of a frequencybelow the' midfrequency. The currents applied to each of the resonantcircuits are rectified and, after rectification, combined.

In practice it has been found that conventional discriminators of thisgeneral type may introduce distortion which was not present in theoriginal currents. It is one of the principal objects of the presentinvention to provide means for eliminating such distortion. Anotherobject is to provide frequency discrimination means in which distortionproduced by undesirable coupling between resonant circuits is eliminatedor minimized. Another object is to provide frequency discriminationmeans in which the distortion produced by an improper ratio of reactanceto resistance is eliminated or minimized.

The invention will be described by referring to the accompanying drawingin which Figure 1 is a schematic diagram of one embodiment of theinvention;` Figure 2 is a schematic diagram of a modification of theinvention; and Figures 3a, 3b, 4a, 4b, 4c and Figure 5 are graphsillustrating the mode of operation.

Referring to Fig. 1, the terminals I, 3 represent the input circuit of afrequency discriminator in which the frequency modulated currents may beamplified, and limited if desired, by a thermionic tube 5. Thethermionlc tube includes an anode 'I which is connected through theprimary 9 of a radio frequency transformer II. The secondary of thetransformer is a part of a circuit I3 resonant at frequency Jli-Af,where f1 equals the mid-frequency and Af is a frequency somewhat largerthan the frequency swing. The primary 9 may be connected in series or inshunt to the second primary I5 of a second radio frequency transformerI1. The second radio frequency transformer has a secondary which is apart of a circuit I9 resonant at frequency fl-Af.

The resonant circuit of the first radio frequency transformer isconnected to a rectifier such as the diode 2|. The rectifier circuitincludes a resistor 23 and a capacitor 25. The second resonant circuitI9 is connected to a second rectier which may be a diode 2-1. The diodeis connected through a resistor 29 and shunt capacitor 3| to theresistor and capacitor of the first diode and to a point intermediatethe low potential ends of the resonant circuits which are connected to-lgether by a conductor 33. The output currents are applied to leads 35,3l which are connected to include the resistors 23, 29.

The thermionic tube 5 includes a screen grid 39 which is connected toground through a bypass capacitor 4| and to +B in the conventionalmanner. It should be understood that certain stray capacitiesunavoidably appear as, for eX- ample, CI between the primary andsecondary of the first radio frequency transformer II, C2 between thewindings of the second radio frequency transformer Il, C3 between theanode and screen grid of the tube 5, and C4 between the anodes of therectifiers 2|, 21. Furthermore, the transformers of necessity havemutual couplings Ml and M2 between their respective windings. Inpractice there is often mutual couplingV M between the resonantcircuits.

The circuit including capacity C3 and the primaries 9 and I5 forms alink circuit which couples the resonant circuits through the magneticcouplings MI and M2 and to some extent through the capacity couplingsCIy C2. Measurements have indicated that the frequency of the linkcircuit should be well removed from that of the two resonant circuitsI3, I9; for example, two and one-half times the resonant frequency ofeither of the resonant circuits I3, I9 is a useful frequency for thelink circuit resonance. Preferably, the link circuit should not respondto the fundamental or any of the harmonics of the signal. Furthermore,measurements have indicated that the sense of the magnetic couplings hasan important bearing on distortion. It has been found that if `the sumofthe several mutual couplings between the resonant circuits isI made.equal to zero or nearly so, th-e distortion is minimized. That is,

where ai, a2, a3, etc., are proportionality constants. In short, theseveral couplings are adlusted so that the resonant circuits I3, I9 aresubstantially without any mutual coupling.

In some practical arrangements, it is necessary Il, I1 included primaryand secondary windings on a core of a half inch diameter. Thesecondaries each included 22 turn of #30 enamelled copper wire spaced bythe diameter of the wire. The primaries each included 28 turns #30enamelled copper wire close wound. Each secondary was three-eighths ofan inch long and spaced one and seven-thirty-seconds of an inch :apartbetween the near edges. The primaries each occupied seven thirty-secondsof an inch in the space between the secondaries. The primaries werespaced one thirty-second of an inch from the secondaries. The effectivesecondary inductance of each secondary was about 10 microhenries; themutual inductance of each primary to secondary was about 2.7microhenries. 'I'he capacity across each secondary was approximately 100micro-microfarads. The rectiiiers were diodes of the type knownl as 6K6.The resistors 23 and 29 were each 120,000 ohms; the capacitors 2l, 35shunting the resistors were 33 micromicrofarads each. The primarycircuit resonated at approximately 10.3 megacycles.

A method of adjusting the circuits is as follows: The primaries areconnected so that the sense of'the mutual coupling of the primaries as alink circuitI is opposite to the direct mutual coupling between thesecondaries. The second step includes short circuiting each of thesecondaries in turn and tuning the other to resonate at the frequenciesfliAf respectively. The third step is to remove the short circuit. Thefourth step is to plot the resonance curves as shown in Fig. 3a or 3b byany suitable means. It the curve corresponds to Fig. 3a, the mutualcoupling is suitably neutralized; if the curve corresponds to Fig. 3b,the mutual couplingjis not neutralized and must be readjusted bychanging the spacing of the secondaries, the phasing of the primaries,or supplementing one of the existing couplings by an additional element.

In Fig. 2, thepreceding thermionic ,tube has been omitted and the inputterminals represented by the reference characters 43, 45. It will beseen that capacity couplings CCI, CO2 and OC3 are used to apply thefrequency modulated currents to the resonant circuits 41, 49. Theresonant circuits are connected to a pair of diode rectiflers which arewithin a single envelope 5I.

The rectifier circuits include resistor-capacity undesirable icouplingshave been eliminated. If the couplings had not been minimized, the

curves might have been represented by the graph of Fig. 3b in which thedistortion is indicated with respect to the curve ,f1-Af. The effect ofsuch distortion is to make the frequency discrimthe distortion will beminimized. In the above equation Q=the reactance to resistace, wo=21rtimes the mid-frequency and W=21r times the difference between theresonant frequencies of the two circuits, for example,

(fl+Af)-(f1-Af) or 2Af It can be shown mathematically that the factor V1.5 is' the optimum value for minimum distortion. Departures from theoptimum value (curve A) produce distortionA effects (curve B) which areshown inthe graph Fig. 5.

Referring to the undesirable effects of mutual magnetic coupling, Fig.4a shows the ideal characteristic of a frequency discriminator as a.broken line 6i. The solid curve 63 shows the p departure which is causedby reversing the phase or sense of one of the magnetic couplings such asMI of Fig. 1. Fig. 4b shows the distortion effects caused by an improperadjustment of the Q of the resonant circuits and Fig. 4c shows thecharacteristic of a. properly adjusted dis-l criminator.

Thus the invention has been described as a frequencydiscriminatonemploying a pair of resonant circuits tuned to frequencyabove and below the mid-frequency of the applied frequency modulatedcurrents. The applied currents are rectified by a pairl of rectifiersand the rectified currents are combined to produce amplitude modulatedcurrents which lmay be detected in the conventional manner. In thepractical arrangement of the device, undesirable mutual couplings areunavoidable. The eiect of these couplings is minimized by adjusting theamount and sense so that 4the resultant coupling approaches zero. Insome arrangements the inherent couplings are employed, and augmented orreversed as required. In other arrangements, additional coupling isadded to neutralize the existing couplings. Furthermore, the Q of theresonant circuits is adjusted to minimize the distortion produced by animproper ratio of reactance to resistance. If the coupling has beenminimized in accordance with the disclosure, the user will find that thecircuits may be adjusted more readily than those of a conventionaldiscriminator in which the circuits react upon each other.

I claim as my invention:

1. A frequency discriminator including a pair of resonant circuitsrespectively responsive to frequencies f1|Af and fl-Af respectively abovand below a reference frequency f1, and further chracterized by a ratioof reactance to resistance equal to (where w=2ff1 and W=21rtf1+nfi21rf1s and 21rAf equals half the radian frequency varianant circuits, aninput circuit for applying frequency modulated currents to said resonantcircuits -and providing mutual coupling therebetween. and magneticcoupling means in said input circuit connected for minimizing thedistortion eiTect of said couplings.

2. A frequency discriminator and detector including a pair of circuitsresonant at frequencies substantially equally above and below apredetermined mid-'frequency of an applied variable frequency signal,each of said circuits including the secondary winding of a couplingtransformer, a primary windingcoupled with each secondary winding, asignal input circuit, connected with said primary windings and forming alink circuit between the secondary windings which resonates at afrequency relatively widely diil'erent from the resonant frequencies ofsaid secondary windings, said -windings having the sense ot the magneticcoupling adjusted for minimizing the effects of mutual and straycapacity coupling `between said circuits, the sense of the mutual cou-vpling of the primary. windings as a link circuit being opposed to thedirect mutual coupling between the secondary windings, means forrectifying the signal output Afrom said secondary windings, and meansfor combining the rectiiled output from said last named means to providethe modulation component oi the applied signal.

NATHANTEL I. KORMAN.

